The present invention relates to a signal transmitting system. More particularly, the invention relates to a signal transmitting system for transmitting signals measured by capacitive sensors in a radiosonde to a surface station via radio transmitter.
A previously known radiosonde signal transmitting system utilizes capacitive sensors which are sequentially connected to the oscillating circuit of an oscillator. The frequency of the oscillator depends upon the quantity being measured; that is, temperature, pressure, or relative humidity. In the known method, the radio transmitter of the radiosonde, which operates at a frequency of 403 MHz or 1680 MHz, is modulated by the frequency of the oscillator or by a frequency derived from the frequency of the oscillator.
Due to the relatively low capacitances of sensors and because of high measuring accuracy requirements, measuring oscillators operating directly on the modulation frequency are out of the question. In the first place, high resistances of 1 to 2 megohms do not meet reliability and stability requirements as far as, for example, temperature, humidity, and aging are concerned. On the other hand, the coils would require such a high stability and such low stray capacitances, that they are out of question in the present application. It is undesirable to connect the sensor capacitor directly to the oscillating circuit of, for example, a 400 MHz oscillator, because of the bulkiness of the sensor capacitors and the high inductances of the wires of the change-over switches.